Our principal objective is to identify, isolate, characterize, and study functions of molecules having primary importance in regulating the animal cell cycle. Significant evidence supports the view that animal cells reach a crucial control point during the G1 phase in the cycle from which they either continue to proliferate or detour to a quiescent state G0. The control event is cytoplasmic. Protein synthesis seems critical in influencing this decision. In particular, we plan to localize the synthesis of regulatory protein (or proteins), both in time during G1 and within the cell. With gels we will seek a protein specifically synthesized in conjunction with the observed kinetics of passage of the cell population through the control event. We will purify this protein, characterize it chemically, and study its properties related to cell cycle events. Its manner of synthesis will be examined in several environments with differing growth situations, in mutant cells with altered cell cycle controls, in cells transformed by DNA or RNA viruses or chemicals, and in revertants of the transformed cells. Cells selectively permeabilized to allow protein entry (without serious disruption of macromolecular synthesis) will be investigated for DNA (S phase) initiation by the protein. Longer-term objectives are to study physiological mechanisms which activate the "on/off switch" in determining this protein's synthesis, and to investigate functions for S phase initiation in vitro.